Metal Organic Chemical Vapor Deposition (MOCVD) is a one of typical vapor-phase growing methods and according to the MOCVD, a group-III metal organic (MO) precursor is gasified and supplied with a carrier gas and a group V gas onto a substrate so that the group-III MO precursor is thermally reacted with the group-V gas on the surface of the substrate to form a film thereon. Since the MOCVD can control the thickness and composition of the film and have excellent productivity, the MOCVD can be widely available as a film-forming technique in the manufacture of semiconductor devices.
An MOCVD apparatus to be employed in the MOCVD includes a reactor, a susceptor disposed in the reactor and gas conduits for flowing reaction gases onto the surface of a substrate disposed on the susceptor. In the MOCVD apparatus, the substrate is disposed on the susceptor and heated at a prescribed temperature while raw material gases such as MO gas and a carrier gas such as nitrogen gas are introduced onto the surface of the substrate through the respective gas conduits so as to conduct the intended thin film-forming process.
In the case that the raw material gases and the like are supplied to the surface of the substrate, group-III gas and group-V gas are introduced into the reactor through a gas distributor disposed opposite to the substrate.
On the other hand, in the case that a plurality of films are stacked by the MOCVD to form a predetermined device, the films are subsequently formed by using the same MOCVD apparatus. Since the compositions of the films are different from one another, however, it may be required that one or more of the raw material gases to be introduced into the reactor through the respective gas conduits are varied remarkably in kind and flow rate per film.
In a proximal gas distribution reactor where a substrate is disposed in the vicinity of a gas distributor or a high velocity rotation reactor where a substrate is disposed away from a gas distributor and rotated at a rotation speed within a range of several hundreds rpm to several thousands rpm, it is required that gases are uniformly supplied through the gas distributor in order to realize the uniformity of a film to be formed on the substrate. With regard to a gas distributor with many holes, the ununiformity of the flow of the gas may occur in the vicinity of the gas distributor due to the blowoff of the gas through the holes of the gas distrubutor, but it is required that the flow of the gas is rendered almost uniform until the gas is flowed to the boundary layer in the vicinity of the substrate.
In order to flow a gas through a gas distributor with a given size uniformly, it is required that an appropriate pressure loss is caused when the gas is passed through the gas distributor and the ununiformity of the gas flow through the gas distributor is not caused due to the gas flow in the upstream of the gas distributor. However, if the kind and flow rate of a raw material gas through the gas distributor are largely changed and for example, the flow rate of the raw material gas becomes low so that the pressure loss of the raw material gas becomes too small when the raw material gas is passed through the gas distributor, the distribution of the flow rate of the raw material gas within the gas distributor becomes large. In contrast, if the flow rate of the raw material gas becomes high, the raw material gas is supplied to the substrate before the flow of the raw material gas through the gas distributor is rendered uniform, causing the ununiformity of a film to be formed on the substrate. Moreover, the ununiformity of the gas flow within the gas distributor and the jet flow of the raw material gas through the gas distributor cause some vortexes in the reactor, resulting in the production of particles in the vicinity of the gas distributor and the production of depositions on the gas distributor.
In this case, the depositions are adhered with the film after or under formation, deteriorating the quality of the film.